Method of forming a thermostatic switch with a narrow operating temperature range

ABSTRACT

The present invention provides a method of forming a thermostatic switch having a calibrated operating temperature range that is defined by a calibrated actuation temperature and a calibrated reset temperature. In accordance with this method a snap action thermostatic switch is provided having a bimetal blade with a snap acting depression. The blade is formed to have a range of actuation temperatures above the desired actuation temperature and a range of reset temperatures below the desired reset temperature. The actuation temperature is calibrated in a heated temperature environment that has an initial temperature that is equal to the desired actuation temperature by a fulcrum-like calibration projection. The reset temperature is then calibrated in the heated temperature environment, after the temperature has been lowered to equal the desired reset temperature. In a preferred embodiment, this is accomplished by bending a terminal strip mounting the bimetal blade, towards a fixed contact of the switch while the blade is in a deformed state.

FIELD OF THE INVENTION

The present invention relates to a thermostatic switch that utilizes asnap acting bimetal blade that, at an actuation temperature deforms andthat, at a reset temperature, returns to an undeformed state toalternately establish circuit open and circuit closed conditions of theswitch. Even more particularly the present invention relates to a methodof forming a thermostatic switch having a calibrated operatingtemperature range in which the differential between the actuation andthe reset temperatures is narrow.

BACKGROUND OF THE INVENTION

Snap acting thermostatic switches have long been used to protect motors,generators, transformers and like electrical components by breakingcontact between the component and a power supply during an elevatedtransient temperature of the ambient and by re-establishing contactbetween the component and the power supply when the ambient temperaturehas cooled to a safe level. Contact is made and broken within the switchby a fixed contact and a movable contact connected to one end of atemperature responsive, snap action, bimetal blade. The blade is mountedin the switch so as to be cantilevered from its other end. There are ofcourse many different switch designs in the prior art. For exemplarypurposes, the fixed contact and the bimetal blade can be mounted on apair of terminal strips that are mounted in a non-conducting case withthe strips insulated from one another. As another example, the blade canbe mounted on a basewall of an electrically conductive can and the fixedcontact can be mounted on an electrically conductive lid that isinsulated from the can. As still another example the switch can have anelongated terminal arm and a terminal lug mounted in the open end of acase in an insulated manner from one another. In such a switch thebimetal blade is cantilevered from the terminal lug and the fixedcontact is connected to an end of the terminal arm that projects intothe case.

The snap action of the bimetal blade is produced by a centrally located,cupped or dish-like portion which can be referred to as a snap actingdepression. When the ambient temperature reaches an actuationtemperature, a sudden reversal of the shape of the depression occurs toproduce a deformed state of the bimetal blade. In the deformed state ofthe bimetal blade, the movable contact is spaced a distance from thefixed contact and the end of the blade, mounting the movable contact, islocated against some point of contact or step on the switch. Dependingupon the switch design, the step can be the terminal strip mounting thebimetal blade, a wall of the case adjacent to the terminal lug that inturn mounts the bimetal blade, or the basewall of the electricallyconductive can mounting the bimetal blade. In any of the switch designs,the spacing of the contacts produces a circuit open condition of theswitch in which contact between a power supply and an electricalcomponent is broken. After a sufficient time has elapsed, and theambient has cooled sufficiently to reach a reset temperature, the snapacting depression reverses to return the bimetal blade to an undeformedstate to produce a circuit closed condition of the switch. In thecircuit closed condition, the movable contact is located against thefixed contact and contact is re-established between the power supply andthe electrical component.

For a variety of reasons, that are well known in the art, a particularbimetal blade design can only be specified as having a range ofactuation temperatures and a range of reset temperatures. In order toinsure the protection of the electrical component from elevated ambienttemperatures, often, the actuation temperature is calibrated to an exactfigure within the range of actuation temperatures. The calibration isperformed on the switch in a heated temperature environment in which theambient temperature is the desired actuation temperature. The blade isthen prestressed by adjustment of a well known fulcrum-like calibrationprojection that is formed on the elongated member of the switch. Thecalibration projection bears against the snap acting depression, whenthe bimetal blade is in its undeformed state, with a sufficient force tocause the depression to suddenly reverse its shape or snap. Even though,when calibrated, such a switch can protect the component from elevatedambient temperatures, the operating temperature differential of theswitch, between the actuation and the reset temperatures, can be far toobroad relative to the safe operating of the component. This is becausethe reset temperature is not calibrated and as such, the resettemperature of the switch can be much lower than the safe operatingtemperature of the component. As can be appreciated, the disadvantage ofthis is that, although the ambient temperature can be at a safe level,the component remains idle until the ambient has cooled to theunnecesarily low reset temperature.

The present invention provides a method of forming a thermostatic switchwith a calibrated operating temperature range in which both theactuation and reset temperatures are calibrated. The operatingtemperature range can therefore be selected to more realisticallyprotect the electrical component, than prior art switches, by producinga calibrated operating temperature range that can be the desired,preferred operating temperature range of the electrical component. Aswill be discussed in greater detail hereinafter, this is accomplished byproviding a blade that has an actuation temperature range that is abovethe desired actuation temperature and that has a reset temperature rangebelow the desired reset temperature. The blade is then prestressed, asdescribed above, to calibrate the actuation temperature. However, unlikethe prior art, the calibrated actuation temperature is below thesupplied range of actuation temperatures for the particular blade. Aftercalibration of the actuation temperature, the blade is also prestressed,when in its deformed state, to upwardly calibrate the reset temperature,above the supplied range of reset temperatures, and towards theactuation temperature.

A switch formed in the manner described above can have a very narrowoperating temperature differential to function with a component having anarrow operating temperature range requirement. It should be pointed outhere that the prior art has provided thermostatic switches with smalloperating temperature differentials. These switches however, haveshallow snap acting depressions of between about 0.0254 mm. and about0.0408 mm. deep, and function more on a creep action of the contactsthan a snap action at the limits of a calibrated operating temperaturerange. Thus, while such prior art switches have a small operatingtemperature differential, they do not have the exactly calibrated,narrow temperature range of a switch formed in accordance with thepresent invention.

SUMMARY OF THE INVENTION

As stated above, the present invention provides a method for forming athermostatic switch with a calibrated, narrow operating temperaturerange. The calibrated operating temperature range is defined by acalibrated actuation temperature and a calibrated reset temperature. Themethod includes providing a thermostatic switch having a fixed contact,a bimetal blade, and a movable contact connected to one end of thebimetal blade. The bimetal blade has a snap acting depression locatedbetween the ends of the bimetal blade. The blade has a range ofactuation temperatures, at which the snap acting depression snaps toproduce a deformed state of the blade, and a range of reset temperaturesat which the snap acting depression snaps back to produce an initialundeformed state of the blade. In accordance with the method of thepresent invention, the blade is formed so that the supplied actuationtemperature range is above the calibrated actuation temperature and thesupplied reset temperature range is below the calibrated resettemperature. The switch also includes means, having an elongated memberand a contact point connected to the member, for mounting the fixedcontact so as to be spaced from the member in the circuit opencondition, facing towards the contact point, and for mounting thebimetal blade in a cantilevered manner from the other of its ends.

An ambient temperature environment is provided for the switch thatinitially has a temperature that is equal to the desired calibratedactuation temperature. In this environment, the blade is in anundeformed state and the movable contact is located against the fixedcontact. A fulcrum-like calibration projection, provided in the member,forcibly bears against the snap acting depression and therebyprestresses the blade to an extent that the snap acting depression snapsto produce the deformed state of the blade. In the deformed state of theblade, the movable contact is spaced a distance from the fixed contactand the other side of the blade is located against a step associatedwith the elongated member. The ambient temperature is then lowered to beequal to the desired calibrated reset temperature. Thereafter, the stepis moved towards the fixed contact to a fixed position in which theblade is prestressed to an extent that the snap action depression snapsback towards the conductive member. Since both the actuation and resettemperatures of the operating temperature range are exactly calibrated,a narrow calibrated operating temperature range can be produced whichcan be the desired range of operating temperatures of the electricalcomponent that the switch is designed to protect. Means are providedsuch that, when the step is repositioned, the fulcrum-like memberremains, essentially, stationary.

DESCRIPTION OF THE DRAWINGS In the accompanying drawings

FIG. 1 is a schematic, elevational view of a thermostatic switch in thecircuit closed condition.

FIG. 2 is a schematic, elevational view of a thermostatic switch in acircuit open condition.

FIG. 3 is a schematic, elevational view of a thermostatic switch havinga calibrated operating temperature differential formed in accordancewith the method of the present invention.

FIG. 4 is a schematic, plan view of an embodiment of an elongatedterminal strip used in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic illustration of a thermostatic switch 1. Switch 1can have an elongated, electrically conductive terminal strip 10, afixed contact 20, a bimetal blade 40 and a movable contact 45. A portionof the switch, illustrated by 30, is provided for mounting the fixedcontact 20 so as to be spaced from and to face towards the terminalstrip 10. The bimetal blade 40 can be connected, at one end 42, to themovable contact 45 and connected, at the other end 44, to the terminalstrip 10, in a cantilevered manner, by means of a weld button 43. Asillustrated, the other end 42 of the bimetal blade freely extends alonga side of the terminal strip 10 facing towards the fixed contact 20. Themovable contact 45 faces towards the fixed contact 20. The bimetal bladealso has a well known dish-like or cupped portion to form a snap actingdepression 46 located between the ends 42 and 44. It is important topoint out here that the depression 46 should be between about 0.100 mm.and about 0.130 mm. deep for a switch formed in accordance with thepresent invention to properly function with a snap action. FIG. 1illustrates the bimetal blade 40 in its initial, undeformed state inwhich the movable contact 45 is located against the fixed contact 20 toform a circuit closed condition of the switch 1. A well knownfulcrum-like calibration projection 16 can be formed in the terminalstrip 10 that bears against the snap acting depression 46 of the bimetalblade 40 when in its undeformed state. With reference now also to FIG.2, a circuit open condition of the thermostatic switch is illustrated inwhich the snap acting depression 46 has snapped towards the fixedcontact 20 to produce a deformed state of the bimetal blade 40 in whichthe movable contact is spaced from the fixed contact 20 and the otherend 42 of the blade is located against a contact point connected to theterminal strip 10. The contact point can be a limit stop projection 18located at end 14 of the terminal strip 10. Projection 18 functions in amanner well known in the art to prevent welding of the blade 40 toterminal strip 10 by electrical arcing between the contacts. Projection18 is optional, however, and the contact point could simply be thesurface of terminal strip 10 opposite to the fixed contact 20.

Bimetal blades, such as illustrated by blade 40, are normallymanufactured by pressing, under great pressure, two metals havingdifferent coefficients of expansion. The snap acting depression, such asillustrated by 46 is formed in a subsequent manufacturing step. Eventhough care is taken in the manufacture of bimetal blades to produceuniform operating characteristics for a particular blade design, thereexist subtle differences in the structure of any two blades of aparticular design. As a result, as stated previously, only a range ofactuation temperatures, in which the snap acting depression 46 snapstowards the fixed contact 20 to produce the deformed state of the blade40 and only a range of reset temperatures, in which the snap actingdepression 46 snaps back towards the terminal strip 10 to return theblade to its undeformed state, can be specified for a particular bladedesign. The blade selected for the method of the present inventionshould have a range of actuation temperatures, specified for theparticular blade, that is above the desired calibrated actuationtemperature. Additionally, the blade should have a range of resettemperatures, specified for the particular blade, that is below thedesired, calibrated reset temperature.

In accordance with the present invention the fulcrum-like calibrationprojection 16, formed in the terminal strip 10, forcibly bears againstthe snap acting depression 46 when the bimetal blade 40 is in itsdeformed state with a sufficient force to prestress the bimetal blade40. In accordance with the present invention, but unlike the prior art,the snap acting depression 46 is prestressed by projection 16 to anextent that the calibrated actuation temperature of the blade 40 islowered below the range of actuation temperatures for a particulardesign of the bimetal blade 40. This is accomplished by heating theswitch 1 in an ambient temperature environment, known well in the art,that has an initial temperature that is equal to the desired actuationtemperature. The projection 16 is then formed to bear against the snapacting depression 46 to cause depression 46 to snap towards the fixedcontact 20. With reference to FIG. 3, after the actuation temperature isexactly calibrated, as described above, the present invention alsocontemplates calibrating the reset temperature. This is accomplished bylowering the temperature of the ambient temperature environment to atemperature that is equal to the desired reset temperature; and,thereafter, moving the contact point (projection 18 of the preferredembodiment) towards the fixed contact 20 to a fixed position in whichthe depression 46 snaps towards the terminal strip 10. In the preferredembodiment this movement step is accomplished by simply bending theterminal 10 towards the fixed contact 20, while making certain that theprojection 16 remains, essentially, stationary.

It is understood that the method of the present invention could haveapplication to many different thermostatic switch designs. In thisregard, it is only required that the switch design incorporate anelongated member. For instance, terminal strip 10 and switch portion 30could be a pair of spaced terminal strips, insulated from one anotherand mounted in an electrically conductive case. In such an embodiment,terminal strip 10 would be the elongated member. Alternately, terminal10 can be the basewall of an electrically conductive can and switchportion 30 could be a lid of the can. The elongated member would be thebasewall of the can. In fact, this latter embodiment could be the switchdesign disclosed in U.S. Pat. No. 3,430,177, which is herebyincorporated by reference. Moreover, as stated previously, the switchportion 30 could be an elongated terminal arm, cantilevered from one endof a case and connected, at the other end, to the fixed contact 20. Insuch a switch, the blade 40 would also be cantilevered from a terminallug mounted in the end of the case and insulated from the terminal arm.As can be appreciated, the elongated member of such a switch would be awall of the case and as such, the bimetal blade 40 would not beconnected directly to the elongated member. In any embodiment, thecalibration projection 16 could be formed, in a manner known well in theart, by dimpling the elongated member at a location adjacent to the snapaction depression 46. In the embodiment disclosed in U.S. Pat. No.3,430,177, the bending step could be accomplished by deforming the canat a location near the limit stop projection, such as illustrated hereinby projection 18. As would occur to one skilled in the art, otherpossible means of prestressing are possible.

One means to assure that projection 16 remains essentially stationarywhen end 14 is bent toward stationary contact 20 is illustrated in FIG.4. In this embodiment, the end 14 is lanced along the lines 50, 51 toform a tongue 52 which mounts projection 18. In calibrating the resettemperature, only the tongue 52 is bent upwardly, thus leavingprojection 16, essentially, in its original position. Further, andwithout lancing, end 42 could, initially, be located in a portion higherthan illustrated, so that very little movement would be needed tocalibrate the reset temperature, again leaving projection 16,essentially in its original position.

Thermostatic switches can be formed in accordance with the method of thepresent invention to have an operating temperature differential ofbetween about 25° C. and about 50° C. Experimentally, a switch has beenformed having an operating temperature differential as narrow as 10° C.Additionally, it is possible to form a switch having an operatingtemperature range slightly above room temperature. In such anapplication, the bimetal blade should be formed with a possible range ofreset temperatures that are below room temperature so that the resettemperature can be upwardly calibrated above room temperature. By way ofan example, a thermostatic switch can be formed in accordance with thepreferred embodiment of the present invention that is designed tooperate at between about 125° C. and about 150° C. In accordance withthe present invention a bimetal blade 40 is provided with an actuationtemperature range of between about 155° C. and about 190° C. and a resettemperature range of between about 60° C. and about 90° C. Afulcrum-like calibration projection, such as 16, is formed in theterminal strip 10 as described above, to reduce and calibrate theactuation temperature to about 150° C. Thereafter, in accordance withFIG. 3 and the above description, the terminal 10 is bent to raise andcalibrate the reset temperature to about 125° C. As can be seen, thereset temperature of the switch 1 approaches the actuation temperatureand the operating temperature differential is only 25° C. As also can beseen, the maximum operating temperature differential of such a switchthat could be expected without a formation of the switch in accordancewith the present invention, would be about 65° C. As another example, athermostatic switch can be formed to operate with an actuationtemperature of about 125° C. and a reset temperature of about 100° C. Insuch a switch, a bimetal blade 40 is selected that has a range ofactuation temperatures between about 155° C. and about 190° C. and arange of reset temperatures of about 60° C. and about 80° C. An exampleof a switch that can be formed to have an operating temperature rangeslightly above room temperature is a switch having an actuationtemperature of about 50° C. and a reset temperature of about 35° C. Inthis switch, a bimetal blade 40 is selected that has a range ofactuating temperatures of between about 60° C. and about 90° C. and arange of reset temperatures of between about 15° C. and about 25° C.

While the present invention has been described by reference to anillustrated preferred embodiment, the invention should not be consideredas so limited but only as limited as set forth in the appended claims.

I, claim;
 1. A method of forming a thermostatic switch with a calibratedoperating temperature range that is defined by a calibrated actuationtemperature and a calibrated reset temperature, said methodincluding:providing a thermostatic switch having,a fixed contact, abimetal blade having a snap acting depression located between the endsof said bimetal blade, a range of actuation temperatures at which saidsnap acting depression snaps to produce a deformed state of said bladeand a range of reset temperatures at which said snap acting depressionsnaps back to return said blade to an initial undeformed state, saidblade being formed so that said range of actuation temperatures is abovesaid calibrated actuation temperature and said range of resettemperatures is below said calibrated reset temperature, a movablecontact connected to one of said ends of said bimetal blade, and means,having an elongated member and a contact point connected to said memberfor mounting said fixed contact so that said fixed contact is spacedfrom said member and faces towards said contact point, and for mountingsaid bimetal blade in a cantilevered manner from the other of its saidends so that when said blade is in its said deformed state, said movablecontact is spaced from said fixed contact and said other end of saidblade is located against said contact point and when said blade is inits initial, undeformed state, said movable contact is located againstsaid fixed contact; providing an ambient temperature environment forsaid switch that is initially equal to said calibrated actuationtemperature; providing a fulcrum-like calibration projection in saidmember to forcibly bear against said snap acting depression and tothereby prestress said blade to an extent that said snap actiondepression snaps to produce said deformed state of said blade;decreasing said ambient temperature environment to be equal to saidreset temperature; and moving said contact point towards said fixedcontact to a fixed position in which said blade is prestressed to anextent that said snap acting depression snaps back to produce saidinitial undeformed state of said blade.
 2. The method of claim 1 whereinsaid contact point is moved by bending said elongated member towardssaid fixed contact.
 3. The method of claim 2 wherein said fulcrum-likeprojection is a calibration dimple formed in said elongated member. 4.The method of claim 1 wherein said fulcrum-like projection is acalibration dimple formed in said elongated member.
 5. The method ofclaim 2 where only a lanced portion of said elongated member is benttoward said fixed contact.